1. Field of the Invention
The invention is in the field of programmable memory cells for random access memories (RAM) and relates to a programmable magnetoresistive memory cell and a RAM memory device comprising such a memory cell.
2. Description of the Related Art
Magnetoresistive Random Access Memory (MRAM) is a non-volatile memory technology that has, for example, been disclosed in U.S. Pat. No. 5,640,343. According to this technology, memory cells include a magnetoresistive element, for example, an anisotropic magnetoresistive (AMR) element, a metallic multilayer element exhibiting the giant magnetomagentoresistance (GMR) effect, or a multilayer element with a magnetic tunnel junction (MTJ) exhibiting the Tunneling Magnetoresistance (TMR).
Because of certain restraints of AMR and GMR memory cells, currently MTJ based technologies are preferred and MRAM devices on the market incorporate MTJ memory cells.
Information bits in both GMR and TMR based memory cells are defined by the relative orientation of the magnetization of two distinct ferromagnetic layers separated by a spacer layer in a memory cell. In a GMR cell, the spacer layer is a non-magnetic metal, whereas, for TMR cells, the spacer layer is electrically insulating and constitutes a tunneling barrier for electrical currents from one ferromagnetic layer to the other one. For example, a logical “1” may correspond to a configuration where the magnetization of the two ferromagnetic layers is parallel, whereas the logical “0” may be defined by the configuration where the magnetization is antiparallel, or vice versa.
The state of the memory cell is determined (“read” operation) by measuring the electrical resistance perpendicular to the layer plane (TMR or GMR based memory cells) or along the layer plane (GMR based memory cells).
Of the two ferromagnetic layers, one is caused to have a well-defined magnetization direction. In this “hard” magnetic layer, the magnetization may be lastingly influenced at most by a very high magnetic field and a field higher than applied during normal operation. For example, the hard magnetic layer may be coupled to an antiferromagnetic (AF) layer. This hard magnetic layer is often also called the “fixed ferromagnetic” layer or “pinned ferromagnetic layer”. The other one of the two ferromagnetic layers has a substantially lower effective coercivity (i.e. the coercivity including influences of the layer's environment such as of pinning etc.) and is called the “soft ferromagnetic layer”.
For a “write” operation, the magnetization of the soft ferromagnetic layer has to be switched. To this end, several approaches have been proposed. A first and most simple approach relies on the generation of a magnetic field sufficient to change the magnetization direction of the soft ferromagnetic layer by causing electrical currents to flow through two perpendicular contact lines above and below the memory cell. This approach is advantageous in being the most simple, however, it requires the coercivity of the soft ferromagnetic layer to be very well defined and, with increasing scaling down in size, the probability of false writes of neighboring memory cells increases.
Other approaches for “write” operations have been proposed, including the “toggle mode” that also requires the application of a magnetic field and the “spin torque transfer”, but they also suffer from drawbacks. Especially, all proposed “write” operations require relatively large write currents. The drawbacks of the “write” operations may be one of the reasons why MRAM devices, although developed more than ten years ago, so far have only been a niche product on the market.
It is therefore desirable to provide an MRAM memory cell overcoming drawbacks of prior art MRAM memory cells and enable improved “write” operations compared to prior art MRAM memory cells.